The present invention relates to a position measuring system for measuring the relative position of two objects, which includes a graduation scanned by a scanning unit to generate at least one periodic analog signal.
In photoelectric measuring systems for measuring the relative position of two objects (such as the relative position between a slidepiece and a bed of a machine tool) a graduation carrier is provided which includes a graduation and is connected with one of the two objects. A scanning unit is connected to the other of the two objects to scan the graduation. The scanning unit includes an illuminating arrangement and a scanning plate which can for example define two scanning fields, the graduations of which are offset with respect to one another by a quarter of the graduation period of the graduation and which agree precisely with the graduation of the graduation carrier. In addition, separate photosensitive elements are allocated to each of the scanning fields. In measuring systems built according to the transmitted light principle, the similar graduations of the scanning plate and the graduation carrier are made up of alternating photopermeable and photoimpermeable strips having a longitudinal extent oriented transversely to the measuring direction. The strips follow one another in alternation in the measuring direction (the longitudinal extent to the graduation carrier). The light flux generated by the illuminating arrangement which passes through the graduations of the graduation carrier and the scanning plate falls upon the respective photosensitive elements and is modulated by the graduations during relative movement between the two objects. The two photosensitive elements aligned with respective ones of the scanning fields generate respective periodic analog signals which are phase-shifted by 90.degree. with respect to one another. These analog signals are applied as inputs to an evaluating arrangement which generates position measuring values therefrom. The phase displacement between the two analog signals permits the direction of measurement to be determined and also allows interpolation of the graduation.
The period of the analog signals generated by the photosensitive elements is determined by the graduation period or grid constant of the graduation of the graduation carrier. The graduation period is determined by the widths of the photopermeable and the photoimpermeable strips in the measuring direction. During relative movement between the scanning unit and the graduation of the graduation carrier, each scanned graduation period is detected by a counting pulse and displayed as a position measuring value.
In general, the periodic analog signals generated from the graduation of the graduation carrier during scanning are not purely sinusoidal in wave form. Rather, these periodic analog signals are typically affected with harmonics as a consequence of (1) spacing fluctuations between the graduation surfaces of the scanning plate and the graduation carrier, or (2) imprecisions in the graduations themselves caused for example by differing spacings of the photopermeable and photoimpermeable strips, or by an edge vagueness of the strips. In order to preserve the analog signals substantially free of harmonics, relatively high demands must be placed on the maintenance of an optimal spacing between the graduation surfaces of the scanning plate and the graduation carrier during the scanning operation. Furthermore, if exact position measuring values are to be formed for each graduation period and exact subdivision of the graduation period is to be accomplished by interpolation for further measuring accuracy, the analog signals obtained from the graduation must be free of harmonics. The formation of interpolation values, for example by means of a computer, is described in DE-OS No. 27 29 297.
Measuring systems utilizing relatively large grid constants (for example 0.1 mm) often generate triangular or trapezoidal analog signals, which by their nature are always affected by harmonics.
German Patent DE-PS No. 19 41 731 discloses a photoelectric length measuring system in which a frequency filter diaphragm with a sinusoidal permeability course is provided to allow recovery of a harmonic-free analog signal during the scanning of the graduation of a graduation carrier. This measuring system presents the disadvantage that a special frequency filter diaphragm must be produced and installed. Furthermore, the field of this frequency filter diaphragm must be completely transilluminated and scanned, which is not always feasible as a practical matter, particularly when scanning in the reflection measuring process. Moreover, the sinusoidal contour of the frequency filter diaphragm causes a considerable reduction in light flux.